Aromatic sulfide polymers, ranging in consistency from viscous liquids to crystalline solids, are known in the art. While such polymers exhibit desirable properties, for many commercial applications, such as molding compositions, the unmodified (i.e., virgin) polymers normally possess a relatively high melt flow, e.g., above about 4,000 grams/10 minutes, which inhibits their use in many applications. For example, when exposed to processing temperatures above their melting point, the virgin polymers require extensive processing times or special apparatus for thin film processing. The high melt flow (low viscosity) of the virgin arylene sulfide polymers makes it difficult to handle these virgin resins by conventional molding practices. Since aromatic sulfide polymers possess many desirable properties which make them extremely useful, it would be advantageous to improve the processability of the polymers without materially affecting any of their desirable properties.
One technique for achieving the improvement in processability is demonstrated in U.S. Pat. No. 3,793,256, henceforth Patent '256. Specifically, Patent '256 discloses, among other things, a method to improve processabilty of a polymeric resin by oxidatively curing the virgin polymer. Since this oxidative curing process comprises exposing of the virgin resin, preferably in particulate form, to an oxidizing atmosphere, while being heated to a temperature below the resin's melting, this process is often referred to as "solid state" curing. Patent '256 further discloses that the oxygen content in air, (i.e., approximately 20 volume %) is preferred to achieve the desirable results of the oxidative curing process disclosed therein. Therefore, since the publication of Patent '256, air has generally been used as the oxidizing atmosphere to cure a virgin poly(arylene sulfide) resin. Conversely, if such a polymeric resin is to be employed for the purposes of, for example, encapsulations and/or coatings, the resin must have its melt flow value increased.
It should be noted that the term "curing", with respect to poly(arylene sulfide) polymers, is not necessarily synonymous with the meaning of that term when applied to other polymers, such as in the "curing" of natural or synthetic rubber with sulfur as the vulcanizing agent, or the curing of unsaturated polyester compositions with styrene as the crosslinking agent. The complete curing of such rubber or unsaturated polyester compositions results in an irreversible conversion from the thermoplastic state to the thermal set state. The curing of poly(arylene sulfide) resins, on the other hand, differs in that these resins are thermoplastic, both before and after such curing, although not necessarily to the same degree. Thus, the curing of PAS resins, and objects formed therefrom, produces changes in extrudability, toughness, and other important properties.
While, for certain commercial applications, it is desirable to decrease the melt flow (increase melt viscosity) of a virgin poly(arylene sulfide) resin, it is equally desirable to have a resin which is thermally stable when in the melted phase. In other words, it is desirable to have a polymeric resin whose melt viscosity does not substantially change while the resin is held at a temperature above its melting point for a given period of time. The reason thermal stability is such an important factor is that in many commercial applications, cured polymeric resins often undergo at least one melt processing step. A typical example of a commercial processing procedure is when a cured resin is first made into a polymer composition by adding to the melted resin various fillers such as glass, fibers, and/or fiberglass. The polymer composition is then often sent to be processed into a final and/or intermediate molded product. This latter processing step often requires that the polymer composition also be heated to a temperature above the melting point of the resin used in its preparation.
Due to inherent physical restraints of commercial processing equipment, it is generally necessary to feed, into the processing equipment, resinous material having a melt viscosity within a specifically narrow range. As illustrated above, the resinous material was exposed to temperatures above its melting point at two different occurrences. Therefore, in that example, if the thermal stability of the cured resin was low (i.e., melt viscosity changed greatly as the resin was maintained in a melted phase), it would be difficult, if not impossible, to determine what its initial melt viscosity should be in order for its final melt viscosity to ultimately fall within the specified range. If, on the other hand, the cured resin is thermally stable in the melt, it will be exceedingly easier to synthesize a resin which will retain the desired flow properties. Accordingly, one object of this invention is to provide a process for preparing thermally stable cured arylene sulfide polymeric resins.
Another object of this invention is to produce polymeric compositions from arylene sulfide resins treated by the novel process of this invention.
Yet a further object of this invention is to make a molded article made from either an arylene sulfide resin, cured by the process of this invention, or a polymeric composition made by using the novely cured resin.
Other objects, aspects, and advantages of this invention will be apparent to those skilled in the art upon reading the specification and appended claims which follow.